Biogas from Energy Crops
There are many potential energy crops, which may be suitable for biogas production including: sugarcane, sorghum, napier grass, as well as, woody crops (tree crops). The best crops should have low fertility requirements, and low energy costs for planting and harvesting. Further, ethanol production from an energy crop will produce large volumes of stillage wastewater, which can be converted to biogas. Also, the production of biodiesel from oil crops produces a glycerol wastewater that also may be converted to biogas. Several research programs investigated energy crops (aquatic and marine plants, grasses, and woods) coupled with anaerobic digestion for generation of renewable substitute natural gas. These programs integrated research on crop production and harvesting, conversion to methane by anaerobic digestion, and systems analysis. In a generalized scheme for anaerobic digestion, feedstock is harvested or collected, coarsely shredded, and placed into a reactor which has an active inoculum of microorganisms required for the methane fermentation. A conventional reactor is mixed, fed once or more per day, heated to a temperature of 35oC, and operated at a hydraulic retention time of 20 to 30 days and loading rate of 1.7 kg VS (organic matter as ash-free dry weight) m3 d-1 (0.1 lb VS ft -3day-1). Under these conditions, about 60% reduction in organic matter is achieved corresponding to a methane yield of 0.24 m3 per kg (4.0 ft3 per lb) VS added. The biogas composition is typically 60% methane and 40% carbon dioxide with traces of hydrogen sulfide and water vapor. Solid residues may be settled and/or dewatered by other means and used as a compost. The product gas can be used directly or processed to remove carbon dioxide and hydrogen sulfide.
Methane from biomass and waste - a program review.
TIDE (Teri Information Digest on Energy) 2(1):1-20 (1992). [PDF]